The present invention relates to a process for the electrochemical roughening of aluminum for use in printing plate supports. The process is performed by means of an alternating current in an electrolyte comprising chloride ions and ammonium ions. The present invention also relates to a printing plate support produced by this process.
Printing plates, which is used here to refer to offset-printing plates within the scope of the present invention, usually comprise a support and at least one radiation-sensitive (photosensitive) reproduction layer arranged thereon. The reproduction layer is applied to the support either by the user (in the case of plates which are not pre-coated) or by the industrial manufacturer (in the case of pre-coated plates).
As a layer support material, aluminum or alloys thereof have gained general acceptance in the field of printing plates. In principle, it is possible to use these supports without modifying pretreatment; however, the supports are generally modified in or on their surfaces, for example, by a mechanical, chemical and/or electrochemical roughening process (sometimes also referred to in the literature as graining or etching), a chemical or electrochemical oxidation process and/or a treatment with hydrophilizing agents. In modern continuously working high-speed equipment employed by the manufacturers of printing plate supports and/or pre-coated printing plates, a combination of the aforementioned modifying methods is frequently used, particularly a combination of electrochemical roughening and anodic oxidation, optionally followed by a hydrophilizing step.
Roughening is, for example, carried out in aqueous acids, such as aqueous solutions of HCl or HNO.sub.3, or in aqueous salt solutions, such as aqueous solutions of NaCl or Al(NO.sub.3).sub.3, using an alternating current. The peak-to-valley heights (specified, for example, as mean peak-to-valley heights R.sub.z) of the roughened surface, which can thus be obtained, are in the range from about 1 to 15 .mu.m, particularly in the range from 2 to 8 .mu.m. The peak-to-valley height is determined according to DIN 4768 (in the October 1970 version). The peak-to-valley height R.sub.z is then the arithmetic mean calculated from the individual peak-to-valley height values of five mutually adjacent individual measurement lengths.
Roughening is, inter alia, carried out in order to improve the adhesion of the reproduction layer to the support and to improve the water/ink balance of the printing form which results from the printing plate upon irradiation (exposure) and developing. By irradiating and developing (or decoating, in the case of electrophotographically-working reproduction layers), the ink-receptive image areas and the water-retaining non-image areas (generally the bared support surface) in the subsequent printing operation are produced on the printing plate, and thus the actual printing form is obtained. The final topography of the aluminum surface to be roughened is influenced by various parameters. By way of example, the following passages from the literature supply information about these parameters:
The paper "The Alternating Current Etching of Aluminum Lithographic Sheet", by A. J. Dowell, published in Transactions of the Institute of Metal Finishing, 1979, Vol. 57, pages 138 to 144, presents basic comments on the roughening of aluminum in aqueous solutions of hydrochloric acid, based on variations of the following process parameters and an investigation of the corresponding effects. The electrolyte composition is changed during repeated use of the electrolyte, for example, in view of the H.sup.+ (H.sub.3 O.sup.+) ion concentration (measurable by means of the pH) and in view of the Al.sup.3+ ion concentration, with influences on the surface topography being observed. Temperature variations between 16.degree. C. and 90.degree. C. do not influence changes until temperatures are about 50.degree. C. or higher, the influence becoming apparent, for example, as a significant decrease in layer formation on the surface. Variations in roughening time between 2 and 25 minutes lead to an increasing metal dissolution with increasing duration of action. Variations in current density between 2 and 8 A/dm.sup.2 result in higher roughness values with rising current density. If the acid concentration is in a range from 0.17 to 3.3% of HCl, only negligible changes in pit structure occur between 0.5 and 2% of HCl, whereas below 0.5% of HCl, the surface is only locally attacked, and at the high values, an irregular dissolution of aluminum takes place. An addition of SO.sub.4.sup.2- ions or Cl.sup.- ions in the form of salts (e.g., by adding Al.sub.2 (SO.sub.4).sub.3 or NaCl) can also influence the topography of the roughened aluminum. Rectification of the alternating current shows that, obviously, both half-wave types are necessary to obtain a uniform roughening.
The use of hydrochloric acid as an electrolyte in the roughening of aluminum substrates is thus to be considered as being basically known in the art. Graining can be obtained, which is appropriate for lithographic plates and is within a useful roughness range. In pure hydrochloric acid electrolytes adjustment of an even and uniform surface topography is difficult and it is necessary to keep the operating conditions within very close limits.
The influence of the electrolyte composition on the quality of roughening is, for example, also described in the following publications:
German Offenlegungsschrift No. 22 50 275 (=British Patent Specification No. 1,400,918) specifies aqueous solutions containing from 1.0 to 1.5% by weight of HNO.sub.3 or from 0.4 to 0.6% by weight of HCl and optionally from 0.4 to 0.6% by weight of H.sub.3 PO.sub.4, for use as electrolytes in the roughening of aluminum for printing plate supports by means of an alternating current,
German Offenlegungsschrift No. 28 10 308 (=U.S. Pat. No. 4,072,589) mentions aqueous solutions containing from 0.2 to 1.0% by weight of HCl and from 0.8 to 6.0% by weight of HNO.sub.3 as electrolytes in the roughening of aluminum with an alternating current.
Additives used in the HCl electrolyte serve the purpose of preventing an adverse local attack in the form of deep pits. The following additives to hydrochloric acid electrolytes are, for example, described:
in German Offenlegungsschrift No. 28 16 307 (=U.S. Pat. No. 4,172,772): monocarboxylic acids, such as acetic acid,
in U.S. Pat. No. 3,963,594: gluconic acid,
in European Patent Application No. 0 036 672: citric acid and malonic acid and
in U.S. Pat. No. 4,052,275: tartaric acid.
All these organic electrolyte components have the disadvantage of being electrochemically unstable and decompose in the case of a high current load (voltage).
Inhibiting additives, for example, phosphoric acid and chromic acid as described in U.S. Pat. No. 3,887,447 or boric acid as described in German Offenlegungsschrift No. 25 35 142 (=U.S. Pat. No. 3,980,539) have the disadvantage that there is often a local breakdown of the protective effect and individual, particularly pronounced, pits can form in these places.
Japanese Patent Application No. 55-17580 describes roughening by means of an alternating current in a composition comprising hydrochloric acid and an alkali-metal halide to produce a lithographic support material.
German Pat. No. 16 21 115 (=U.S. Pat. Nos. 3,632,486 and 3,766,043) describes roughening by means of a direct current in dilute hydrofluoric acid, whereby the web is switched such that it forms the cathode.
German Pat. No. 120 061 describes a treatment for generating a hydrophilic layer by the application of electric current. The treatment can also be performed in hydrofluoric acid.
Japanese Patent Application No. 55-21101 describes the production of a capacitor film. In the process, roughening is first carried out in an electrolyte comprising from 0.3 to 1.5% of hydrochloric acid and from 15 to 25% of ammonium acetate using an alternating current (at 200 to 400 C/dm.sup.2). Electrolysis is then continued in HCl using a pulsed current.
German Auslegeschrift No. 22 18 471 (=U.S. Pat. No 3,775,116) describes the addition of amines, in concentrations ranging from 0.05 to 5%, as an anticorrosive agent to a 1 to 3% strength hydrochloric acid electrolyte used for the production of printing plate supports. As a secondary pickling agent, at least one of the substances including magnesium chloride, aluminum chloride, zinc chloride and ammonium chloride is used, in a total chloride concentration of up to 8% by weight, based on the total weight of the electrolyte.
Since the secondary pickling agents are weighted as equivalent, the examples clearly show that the surface is rendered uniform by the anticorrosive agents specified, and not by a specific action, for example, of the ammonium ions.
It is also shown by the examples that these anticorrosive agents produce an increase in peak-to-valley height, as compared with pure hydrochloric acid electrolytes.
Japanese Patent Application No. 55-34406 claims 0.3 to 1.5% of HNO.sub.3 and 1 to 30% of citric acid, in addition to 15 to 25% of ammonium acetate.
However, a treatment of this kind in electrolyte systems with a pH exceeding 4.5 leads to surface structures which are coarsely pitted and/or do not show an overall roughening and which are entirely unsuited for lithographic purposes (cf. Comparative Examples C41 to C48). Contrary to surface enlargement which is desired for application in capacitors, roughening of printing plate supports serves to produce layer anchoring and water/ink balance and must, therefore, be very homogeneous and free from pits.
Another known possibility for improving the uniformity of electrochemical roughening comprises a modification of the type of electric current employed, including, for example,
using an alternating current, in which the anodic voltage and the anodic coulombic input are higher than the cathodic voltage and the cathodic coulombic input, according to German Offenlegungsschrift No. 26 50 762 (=U.S. Pat. No. 4,087,341), the anodic half-cycle period of the alternating current being generally adjusted to be less than the cathodic half-cycle period; this method is, for example, also referred to in German Offenlegungsschrift No. 29 12 060 (=U.S. Pat. No. 4,301,229), German Offenlegungsschrift No. 30 12 135 (=published UK Patent Application No. 2,047,274) or German Offenlegungsschrift No. 30 30 815 (=U.S. Pat. No. 4,272,342),
using an alternating current, in which the anodic voltage is markedly increased compared with the cathodic voltage, according to German Offenlegungsschrift No. 14 46 026 (=U.S. Pat. No. 3,193,485),
interrupting the current flow for 10 to 120 seconds and reapplying current for 30 to 300 seconds, using an alternating current and, as the electrolyte, an aqueous solution of 0.75 to 2.0 N HCl, with the addition of NaCl or MgCl.sub.2, according to British Pat. No. 879,768. A similar process comprises an interruption of current flow in the anodic or cathodic phase is also disclosed in German Offenlegungsschrift No. 30 20 420 (=U.S Pat. No. 4,294,672).
The aforementioned methods may lead to relatively uniformly roughened aluminum surfaces, but they sometimes require a comparatively great equipment expenditure and, in addition, are applicable only within closely limited parameters.